This invention is generally directed to improvements in television receivers, and particularly to an improved oscillator for use in a television receiver having an electronically tuneable tuner.
Conventional television tuners typically receive a variable DC tuning voltage for altering the characteristics of a varacter diode. As the characteristics of the diode change, the tuner's oscillator oscillates at different frequencies for detecting television signals broadcast over various channels.
A problem with many tuner oscillators is that their output power decreases significantly as they are tuned to frequencies corresponding to lower numbered channels. Consequently, the oscillator injection to the receiver's mixer decreases. This causes the mixer's conversion gain to suffer a corresponding decrease, and also results in a poorer noise figure for the tuner.
Because the power output of conventional tuner oscillators is relatively low at frequencies corresponding to low channel numbers, the mixer is typically a bipolar device capable of operating with low signal injection. However, bipolar mixers normally have poorer cross-modulation characteristics and poorer signal handling capability than is desired. MOSFET type mixers are superior in both respects to bipolar mixers, but they require an oscillator injection signal which is relatively large over the entire band of oscillator frequencies. Hence, MOSFET type mixers are difficult to use with conventional oscillators. These and other problems discussed hereinafter are associated with many conventional electrically tuneable oscillators.